Attention has been attracted to high-output laser light sources with outputs exceeding several W as light sources used for laser processing or laser displays. Semiconductor lasers using gallium arsenide, gallium nitride and the like have been developed in red and blue regions and it has been also studied to realize higher outputs. However, it is still difficult to directly generate green laser light from a semiconductor.
Thus, a method for obtaining green light as a second harmonic by wavelength converting infrared light or the like as a fund amental wave has been generally employed. Specifically, infrared light emitted from a solid-state laser such as a YAG laser or a fiber laser using a fiber doped with a rare-earth element such as Yb or Nd is caused to be incident on a nonlinear optical crystal and green light is obtained by wavelength conversion by the nonlinear optical crystal.
Particularly, a wavelength conversion element formed with a quasi phase matching (QPM) structure using a polarization reversal technology for lithium niobate or lithium tantalate is known to have a large nonlinear optical constant and to be able to obtain green light from infrared light with a high conversion efficiency. Further, by doping a wavelength conversion element with magnesium oxide, it became possible to suppress a change in refractive index (photorefractive) caused by light, which is one of crystal degradations and enable a stable wavelength conversion at ordinary temperatures as disclosed in Non-Patent Literatures 1, 2.
In the case of causing infrared light as a fundamental wave to be incident to generate green light as a second harmonic in a wavelength conversion element composed of lithium niobate doped with magnesium oxide, it is problematic that crystal destruction starts in the second half of a beam path upon generating an output exceeding 2 W although it differs depending on the element. In the case of pulse oscillation with a high peak value, crystal destruction occurs when an average output exceeds 0.5 W.
In a wavelength conversion element composed of lithium tantalate, crystal destruction that occurs at the time of a high output is similarly problematic.
Here, it can be thought to use a plurality of wavelength conversion elements as disclosed in Patent Literature 1 or form a plurality of optical paths in one wavelength conversion element as disclosed in Patent Literature 2, for example, in order to obtain a harmonic with a high output of 5 W. However, regardless of which construction is employed, a green output which can be generated from one optical path is at most 2 W if crystal destruction is considered. Thus, three wavelength conversion elements or optical paths are necessary.    Patent Literature 1: Japanese Unexamined Patent Publication No. H11-271823    Patent Literature 2: Japanese Unexamined Patent Publication No. 2004-125943    Non-Patent Literature 1: Applied Physics letters, 44, 9, 847-849 (1984)    Non-Patent Literature 2: Applied Physics letters, 59, 21, 2657-2659 (1991)